The latest backup is always available to you by entering heroku pgbackups:url
. So, set up a cron job or the equivalent that fetches that URL once a week or once a day.
You could write a rake or Ruby script and call it with Heroku Scheduler (a free addon), or use a different remote machine to pull the backup, or do a shell script and:
curl -O `heroku pgbackups:url`
Here is a Gem that appears to do what you want: https://coderwall.com/p/w4wpvw